A new ischnacanthiform acanthodian Serradentus armstrongi nov. gen. et sp. has been collected from the Middle Devonian (Givetian) Fiskekløfta Member, the upper member of the Tordalen Formation in the Mimerdalen Subgroup of Spitsbergen. The specimen comprises both pairs of upper and lower jaw bones, jaw cartilages, tooth or denticle whorls, dentition cones and spiky denticles. The latter two features have previously only been identified in Early Devonian ischnacanthiforms. The geology is consistent with a depositional environment of a brackish backwater lagoon with an anoxic bottom. The fish probably entered the lagoon during storm action and died due to the storm churning up bottom anoxic waters, or from being trapped and killed by hypersalinity. The fish possibly partially decomposed on the surface before the head detached and sank to the lagoon floor. The lack of scavengers due to the anoxic conditions prevented scattering of the individual elements, with denticles preserved along the labial surface of the dentigerous jaw bone, and jaw cartilage under the jaw bone.